Poliigon.com leaf textures
Mirror UV

Once you unwrapped the geoemetry, I would duplicate the uvmap with the "+" button on the right and then flip the UV islands on the other side of the texture. This way you just have to pick the right coordinate (in cyles for example the UV node as shown below) for two istances of the same texture and mix them with backfacing factor.



Reynante Martinez wrote an article about Double-Sided Texturing in Cycles, you can read it at: http://www.reynantemartinez.com/double-sided-texturing-in-cycles.html.



You can also take advantage of the Vector Mapping node to automatically flip your UV so you'll even not need to duplicate your UV map.